1. Field of the Invention
The present invention relates to a method of making a charge coupled device image sensor, and more particularly to a method of making a charge coupled device image sensor capable of a smear phenomenon.
2. Description of the Prior Art
Recently, solid state image sensors made by a semiconductor integrated circuit technique have been used as photoelectric conversion elements for image pickup devices converting images of objects into electric signals. Such sold state image sensors are generally classified into image sensors of the metal oxide semiconductor (MOS) type in which photo charge generated at a photodiode as a photoelectric conversion element is read out by a MOS transistor and image sensors of the charge coupled device (CCD) type in which photo charge generated at a photodiode is read out by a CCD shift resistor. Over the MOS type image sensors, the CCD type image sensors have an advantage of reduced noise, in that the photo charge generated at the photodiode is read out efficiently according to a clock signal.
Referring to FIG. 1, there is shown a sectional view illustrating of a structure of a general CCD image sensor. As shown in FIG. 1, the image sensor comprises a plurality of light receiving elements 11 each comprising a photodiode for generating photo charge in proportion to incident light and accumulating it, a plurality of vertical charge transfer elements 12 each comprising a CCD shift resistor for vertically transferring the photo charge accumulated in each corresponding light receiving element 11 according to a predetermined clock signal, a horizontal charge transfer element 13 comprising a CCD shift resistor for receiving signal charges from the vertical charge transfer elements 12 in a parallel manner and transferring them horizontally, and a sensing amplifier 14 for amplifying each signal charge transferred from the horizontal charge transfer element 13 and outputting it at an output terminal OUT.
FIG. 2 is a cross-sectional view taken along the line A--A' of FIG. 1, showing the sectional structures of each light receiving element 11 and each vertical charge transfer element 12 in the CCD image sensor.
As shown in FIG. 2, the structure of conventional CCD image sensor comprises a n type semiconductor substrate 21 and a p type well 22 formed over the semiconductor substrate 21. The well 22 has a first n.sup.+ type impurity region 23 forming a pn junction together with the well 22 to constitute a photodiode as the light receiving element 11, and a second n.sup.+ type impurity region 24 serving as a channel region of a vertical CCD shift resistor as the vertical charge transfer element 12. Over the well 22 is coated a gate oxide film 25 and a gate 26 as transfer electrodes of the vertical charge transfer element 12. A gate insulating film 27 is formed over the exposed surfaces of the gate oxide film 25 and gate 26. Over the gate insulating film 27, a photo shield layer 28 is formed which is made of a metal and adapted to allow light to enter only the n.sup.+ type impurity region 23 as the light receiving element.
In the conventional CCD image sensor with the above-mentioned structure, the first n.sup.+ type impurity region 23 together with the p type well 22 constitute the photodiode with the pn junction, which photodiode generates a photo charge in proportion to incident light and accumulates it. The accumulated photo charge is transferred to the second n.sup.+ type impurity region 24 according to a clock signal applied to the gate 26.
As mentioned above, the conventional CCD image sensor has the photo shield layer which functions to allow light to enter only the first n.sup.+ type impurity region 23 as the light receiving element and to prevent light from entering the second n.sup.+ type impurity region 24 as the charge transfer element. However, since a part of the incident light may enter inclinedly the first n.sup.+ type impurity region 23, it can reach the p type well 22 disposed around the second n.sup.+ type impurity region 24, thereby causing a noise charge e to generate at the p type well 22. This light is denoted by the reference numeral 29, in FIG. 2. The generated noise charge e is then transferred to the n.sup.+ type impurity region 24 and causes a smear phenomenon making the profile of a picture smear. As a result, there is a problem of a degradation in image quality, in the conventional CCD image sensor.
For avoiding such a smear phenomenon caused by the noise charge generated due to the leakage light, there has been a method wherein p.sup.+ type impurity regions surround the n.sup.+ type impurity regions 23 and 24 functioning as the photodiode and the channel region of vertical CCD shift resistor, respectively. In accordance with the method, the p.sup.+ impurity region surrounding the first n.sup.+ type impurity region 23 has a depth larger than that of the p.sup.+ impurity region surrounding the second n.sup.+ type impurity region 24 so that the generated noise charge is discharged out of the lowermost substrate 21, so as to avoid the smear phenomenon.
Another method for avoiding the smear phenomenon is a method for reducing the thickness of the gate insulating film 27 formed over the light receiving element.
Referring to FIGS. 3A to 3D, there is illustrated a method of making a CCD image sensor which has a gate insulating film with a reduced thickness for avoiding the smear phenomenon.
In accordance with this method, first, over a n type semiconductor substrate 31 is formed a p type well 32, as shown in FIG. 3A. In the p type well 32, n type impurity ions such as As or P are implanted to form n.sup.+ type impurity regions 33 and 34 serving as a photodiode and a channel region of a vertical shift resistor.
Thereafter, a gate oxide film 35 is formed over the resultant entire exposed surface. Over the gate oxide film 35 is deposited a polysilicon film which is, in turn, subjected to a patterning for forming a gate 36 between the impurity regions 33 and 34.
Over the resultant entire exposed surface, a low temperature oxide (LTO) film or a boro-phosphorous silicate glass (BPSG) film is then deposited to form a gate insulating film 37, as shown in FIG. 3B. Over the gate insulating film 37 is formed a photoresist 38 which is then partially removed at its portion disposed over the n.sup.+ type impurity region 33, so as to expose the corresponding portion of gate insulating film 37. Using the remaining portion of photoresist 38 as a mask, the exposed portion of gate insulating film 37 is partially removed by a dry etch method, so as to reduce its thickness. Subsequently, the remaining portion of photoresist 38 is removed.
Thereafter, a metal layer 39 made of, for example, aluminum is deposited over the resultant entire exposed surface, as shown in FIG. 3C. Over the metal layer 39 is coated a photoresist 40 which is, in turn, subjected to a photo etching so that its portion disposed over the n.sup.+ type impurity region 33 is removed for exposing the corresponding portion of metal layer 39.
Using the remaining portion of photoresist 40 as a mask, the exposed portion of metal layer 39 is etched, so as to form a photo shield layer for preventing light from entering regions except for the light receiving element, as shown in FIG. 3D. Finally, the remaining portion of photoresist 40 after the formation of photo shield layer is removed. Thus, a CCD image sensor is obtained.
In the CCD image sensor obtained by the above-mentioned method, the portion of gate insulating film 37 disposed over the light receiving element is partially etched to have a reduced thickness. Accordingly, light does not enter the n.sup.+ type impurity region 34, but enters only the n.sup.+ type impurity region 33 by virtue of the photo shield layer provided by the metal layer 39. As a result, it is possible to prevent the generation of noise charge and thus the smear phenomenon due to the noise charge.
FIGS. 4A and 4B are sectional views for explaining a smear effect depending on the thickness of gate insulating film over the light receiving element.
FIG. 4A is a sectional view of the CCD image sensor in which the insulating film 37 formed on the entire surface of the substrate 31 is partially etched so that its portion disposed over the n.sup.+ type impurity region 33 has a reduced thickness t1. On the other hand, FIG. 4B is a sectional view of a CCD image sensor in which the insulating film 37 is not subjected to an etching and thus has a uniform thickness t2 throughout the entire surface of substrate 31.
Where the thickness t2 of insulating film 37 over the n.sup.+ type impurity region 33 as the light receiving element is large, as shown in FIG. 4B, vertical incident light can not enter the n.sup.+ type impurity region 34 as the charge transfer element by virtue of the photo shield layer 39, but enter only the n.sup.+ type impurity region 33 and serves to generate a photo charge which is a signal charge.
Even though the photo shield layer 39 is provided, however, inclined incident light enters regions other than the n.sup.+ type impurity region 33. Among inclined incident lights, light L2 entering the p type well 32 disposed around the n.sup.+ type impurity region 34 generates a noise charge at a region near the n.sup.+ type impurity region 34. This noise charge is transferred to the n.sup.+ type impurity region 34, thereby causing a smear phenomenon.
In FIG. 4B, the reference character L3 denotes a leakage light which is reflected on the boundary surface defined between the insulating film 37 and the substrate 31. This light L3 causes a multi-reflection and thereby enters the p type well 32 disposed around the n.sup.+ type impurity region 34. As a result, a noise charge is generated in the same manner as mentioned above, thereby causing a smear phenomenon.
On the other hand, where the insulating film 37 has the reduced thickness t1 at its portion disposed over the n.sup.+ type impurity region 33 serving as the light receiving element, as shown in FIG. 4A, inclined incident light is prevented from entering the n.sup.+ type impurity region 33. As a result, it is possible to prevent the multi-reflection of light on the boundary surface defined between the insulating film 37 and the substrate 31 and thus restrain the generation of smear phenomenon.
However, although the CCD image sensor in which the insulating film 37 is partially etched to have a reduced thickness at its portions disposed over the light receiving element shields effectively the leakage light and thus avoids a smear phenomenon, it also has other problems that the thickness of insulating film 37 is difficult to be freely adjusted since the insulating film 37 is dry etched and that the surface of substrate may be damaged if the insulating film 37 is excessively etched.
The damaged substrate surface caused by the excessive etching of insulating film results in an increase in dark current and an increase in white defect, thereby degrading the image quality of image sensor.